1. Field
This disclosure relates generally to electrical isolation structures for integrated circuits, and in particular but not exclusively, relates to electrical isolation structures for image sensors.
2. Background Information
Image sensors typically include pixel arrays that include electrical isolation regions between adjacent pixels. The isolation regions help to electrically isolate or insulate the adjacent pixels from one another.
FIG. 1 is a cross-sectional side view of a simple two-pixel example of a pixel array. The pixel array has a first pixel 102-1 and a second pixel 102-2 formed within a silicon substrate 101. The pixel array also includes a first isolation region 103-1, a second isolation region 103-2, and a third isolation region 103-3. The first isolation region 103-1 may be disposed between the first pixel 102-1 and another pixel (not shown) that is positioned to the left of the first pixel, the second isolation region 103-2 is disposed between the first and second pixels 102-1, 102-2, and the third isolation region 103-3 may be disposed between the second pixel 102-2 and another pixel (not shown) that is positioned to the right of the second pixel.
Each of the isolation regions includes a corresponding doped isolation region 104-1, 104-2, 104-3, and a corresponding deep trench isolation (DTI) 105-1, 105-2, 105-3. The doped isolation regions represent doped regions or wells formed within the silicon substrate that include a dopant of a type that is operable to make doped isolation regions that electrically separate the photogenerated carriers of adjacent pixels. The doped isolation regions begin near the upper surface of the silicon substrate and extend into the silicon substrate to a depth appropriate to help isolate the adjacent pixels.
The DTIs 105 are disposed within the doped isolation regions 104. DTI has been employed relatively recently as an alternative to shallow trench isolation (STI) in an effort to improve the amount of isolation of the adjacent pixels. Each of the DTIs represents a deep trench that has been etched into the silicon substrate and then filled by adding silicon dioxide into the deep trench. The DTIs begin near the upper surface of the silicon substrate and extend part way through the doped isolation regions. As the names implies, DTI is typically significantly deeper than STI.
Over the years the size of pixels in image sensors has decreased significantly. The reduction in the size of the pixels has been motivated in part by factors such as a desire to provide increased image sensor resolution, reduced image sensor size, reduced image sensor manufacturing costs, reduced image sensor power consumption, or the like. Further reductions in the sizes of the pixels are desirable.
One factor that contributes to the size of the pixels and/or the pixel array is the width of the trench isolation (e.g., DTI 105 or STI). The smaller the widths of the trench isolation, the more closely together the pixels may be packed within the pixel array. Accordingly, it would be desirable to reduce the width of the trench isolation. However, reducing the width of the trench isolation tends to be challenging. For one thing, it tends to be challenging to etch a narrow, deep trench into the silicon substrate. For another thing, it tends to be challenging to fill such a narrow, deep trench in the silicon substrate with silicon dioxide without creating voids, stress, or defects. This would be especially the case if the width of the trench isolation were to be less than about 0.3 micrometers (μm).